[Aidan]

No kind of engine needs backpressure. Search the internet for exhaust scavenging and learn all about it. People talking about backpressure really makes me mad. So many people believe in this myth its not funny.

Short answer: Turbo means go with the biggest freest exhaust you can stand.

 

[Aidan]

4 stroke engines do not, but 2 stokes heavily rely on a properly tuned exhaust!

Now, I'll be honest, I don't know much about 2-stroke motors but is it backpressure they're relying on when you tune the exhaust? I would imagine its the same scavenging effect you are looking for in a naturally aspirated 4-stroke, just has much more of an effect on the 2-stroke.

You can tune and exhaust on a 4-stroke naturally aspirated motor... heck, I bet you can even do it on a supercharged motor. The problem is that the turbo screws up any effect you could really have on tuning an exhaust so you might as well go as big as possible

 

[Aidan]

Scavenging, exactly what I was talking about. Its just much more noticeable on a 2-stroke motor than a 4-stroke. It does help to tune the scavenging on a 4-stroke as well, just not on a turbocharged one like these diesels. And to be completely accurate, a diesel is not a 4-stroke motor since it is based on the Diesel cycle and not the Otto cycle.